In the early 1950s the introduction of tungsten carbide cutting elements caused a revolution in rolling cutter drill bits of the above type. Previous bit designs utilised iron or steel rolling cones with cutting elements milled into their surfaces. The life of these milled tooth bits was limited compared to bits with the cutting elements made of sintered tungsten carbide inserts.
Although these inserts greatly improve the drill bit life, their use introduces a new set of design problems. The layout of the cutting elements is more critical, due to the relatively smaller size of the carbide inserts when compared to milled teeth. In addition, the recesses which hold the cutting elements must be arranged so as not to intersect each other below the surface of the rolling cone.
A few very hard formation bit designs feature very dense packing of inserts in cones. The number of inserts which can be employed is limited only by interference of the recesses which hold the cutting inserts. In these bits, the insert row placement upon any one cone is independent of the other two cones, giving the bit designer considerable freedom in row placement. Although this design allows for a very durable cutting structure, the dense packing of inserts and their limited protrusion cause very slow rates of penetration. In this design, the diameters of the rolling cones and the protrusions of the inserts must be sized such that one cutter does not interfere with the adjacent cutters. This design is known as an independently rolling or non-intermeshing bit design. When compared to other three cone drill bit designs, particularly those for drilling soft formations, the reduced cone diameter in a non-intermeshing bit design unacceptably limits bearing size and capacity. Examples of non-intermeshing designs are in U.S. Pat. Nos. 4,056,153, 4,320,808, 4,393,948, and 4,427,081. Non-intermeshed three cone rolling cutter bits are not in common use today.
Most modern three cone insert bits have intermeshed rows of inserts. Although row intermeshing further constrains insert row layout, the bit is still expected to have a long life while maintaining a fast drilling rate. These performance expectations require that the cones be as large as possible within the borehole diameter to provide adequate recess depth for the cutting inserts and the maximum possible bearing size. To achieve maximum cone diameter and still have acceptable insert protrusion, some of the rows of inserts are arranged to protrude into corresponding clearance grooves on adjacent cones. The combined row layout of three intermeshed cutter cones will be sequence of alternating rows from adjacent cones as shown in FIG. 5 of U.S. Pat. No. 4,611,673. The intermesh arrangement allows cutting tips of rows on adjacent cutters to interfit upon bit assembly without interference. Unfortunately, the arrangement limits the conglomerate of the three intermeshing cutters to one operative insert row per track along the borehole in the intermeshed area. Although some rows of inserts near the gauge and at the center of the bit are not intermeshed, the placement of all rows upon the cones is heavily influenced by the placement of the intermeshed rows.
In the drill bit industry there are several different row naming nomenclatures. The nomenclature used herein is similar to that used in U.S. Pat. Nos. 4,611,673 and 4,940,099 and is defined as follows.
Reaming insert rows are located on the portion of the cone closest to the sidewall of the borehole and closely adjacent to the bit body. These inserts act as necessary to ream the already cut full gage diameter of the borehole well above the bottom of the borehole. Reaming rows of inserts are commonly only slightly protruding and non-intermeshing and are of minimal importance in this specification. Reaming insert rows are shown as numeral 32A in U.S Pat. No. 4,940,099.
The row of a cone which first engages the uncut full diameter of the borehole is the gauge row. Most bits have three gauge rows, one row per cone, which redundantly cut gauge at the same area of the formation. The gauge rows are shown as numeral 26 in U.S. Pat. No. 4,611,673. Gauge cutting inserts are located on the cones so as to cut the earth formation adjacent to the hole bottom and often cut a portion of the hole bottom in addition to the gauge. In some bit designs, notably U.S. Pat. No. 3,452,831, several gauge rows of inserts are indicated. Since only the row of a cone which first engages the formation at the gauge of the borehole is the true gauge row, any other rows on the cone which are placed to cut gauge are reaming already cut formation and act as reaming rows.
The intermediate rows of inserts cut the hole bottom. These are the rows on the cones which are most often intermeshed, and are shown as numeral 28 in U.S. Pat. No. 4,611,673.
The nose rows of inserts, shown as numeral 30 in U.S. Pat. No. 4,611,673, are designed to cut near the center of the borehole. These rows can be, but are not always intermeshed.
The rows which cut closer to the center of the borehole than the gauge row (i.e. the intermediate and nose rows) are collectively called the inner rows of the bit.
Drill bits often have a plurality of non-intermeshing rows which redundantly cut along the same track of the formation. As far as the formation is concerned this plurality of rows acts as a single operative row. An operative row is therefore one or more rows of a drill bit which act to cut substantially a single track along the borehole.
By design, each operative insert row is dedicated to cut a specific region of the borehole. The shape (or profile) at the bottom of the borehole is determined by the arrangements of the operative rows of inserts on the bit and the shapes of the cutting inserts. The shape of the borehole has a major influence on the forces imposed on the cutting inserts during drilling and is an important consideration when designing bits for fast penetration and long life. The nomenclature for the various regions of the borehole bottom follows.
At the center of the borehole is the core region. The core is cut by the nose insert rows and is rather easily cut and broken off.
Concentric to the core is the bottom region of the profile. The bottom region is cut by the intermediate rows of the bit. The outer edge of the borehole bottom is cut by the row or rows of inserts on the bit with the greatest cutting diameter with respect to the rotational axis of the cone.
The gage region of the borehole is the cylindrical full diameter surface cut by the gauge and reaming rows of inserts.
The transition region of the borehole is the narrow ring between the outer edge of the borehole bottom and the gauge. An example of a transition region is found in U.S. Pat. No. 2,990,025, FIGS. 2 and 3. The tip of the rows containing the insert indicated as numeral 21 have the greatest radial displacement from the cone's centre of rotation of all other rows of the three cones. This intermediate row, therefore, defines the edge of the hole bottom. The narrow area between this row and the gauge row indicated as numeral 20 is the transition region of the borehole.
In many prior art three cone insert bit designs, and as shown in U.S. Pat. No. 2,774,570 FIG. 1, the rotating cutters have their largest diameter at the gauge insert rows. As a result, both gauge of the borehole and the outermost edge of the hole bottom are drilled by the gauge rows of inserts. This makes the transition of the borehole from the vertical sidewall to the borehole bottom (or corner of the borehole) relatively sharp. A sharp borehole corner, as reported in U.S. Pat. No. 4,231,438, is required so that the bit will maintain a straight drilling path through sloping formations and also helps reduce existing borehole deviation. Even in bit designs utilising different rows for gauge and hole bottom drilling, the corner is still designed to be relatively sharp so that a straight borehole will be drilled. Sharp borehole corners are difficult to cut, however, because of the support lent to the corner by both the borehole wall and the borehole bottom. The insert rows which cut the borehole corner, and particularly the gauge rows, sustain higher forces than any other rows of the bit.
Because of these higher forces, an important design factor for drill bits is the manner in which gauge insert rows are designed. It is important to have as many cutting inserts as possible on the gauge of the bit in order to prolong bit life. For stability of drilling, it is also important that each cone have a gauge row which acts upon the same portion of gauge of the borehole, redundantly. A cone without a gauge row or with a gauge row placed to drill a different portion of the borehole, either closer to or farther from the bottom than the others, will experience different magnitudes and directions of cutting forces. Under certain drilling conditions, this force imbalance can cause the bit's longitudinal axis to orbit about the center of the borehole significantly, a phenomenon called bit gyration. Bit gyration is unacceptable because it causes an uncontrolled hole size to be drilled and it reduces drilling rate.
Modern drill bits must also have row intermeshing to permit high insert protrusions in order to achieve competitive rates of penetration. The constraints of row placement due to intermesh, however, limit the number of operative rows on the bit. Gauge row insert interlocking, as shown in U.S. Pat. No. 2,990,025, has become the accepted manner in which to optimize the row intermeshing of the bit to allow high insert protrusion and still provide an adequate number of cutting inserts for drilling the corner of the borehole. Insert interlocking is the placement of two closely adjacent rows of inserts on the same cone such that each row cuts a different track along the hole bottom, and where the inserts in the rows are alternated to prevent interference between the inserts within the cone. As a consequence, the number of inserts that can be placed on either of the interlocked rows is fewer than the number possible without interlocking. Even though interlocking reduces the number of individual gauge inserts possible on a bit, it facilitates close proximity of adjacent operative rows. Most successful prior art bit designs have three intermeshed cutters with at least one and most often two interlocked gauge rows.
With the advent of modern directional drilling "steerable" drilling systems have become common. Directional drilling has changed the way conventional straight hole drill bits are run and consequently changed the modes of decay of the bits. In particular, accelerated wear and breakage occur on the borehole corner drilling inserts, especially the gauge rows and the closest operative inner row to the gauge. This insert wear and breakage occurs because a bit designed to drill a straight hole experiences higher than normal side forces concentrated upon the gauge and the closest operative inner row to the gauge when forced to drill a curved hole. Because the borehole corner drilling rows have not been designed for directional drilling, sideways acting forces lead to insert breakage. A bit designed to drill a straight hole also places more stress than necessary upon the bit steering mechanism when a curved hole is drilled.